This invention relates to a process for the relaxation of stresses in a plastic sheet material so as to minimize any subsequent tendency to shrink.
When a thermoplastic material is extruded to form a sheet or film, it is often found that subsequent re-heating leads to shrinkage or curling as a result of the relaxation of stresses built into the sheet or film during production. The extent to which this happens is dependent largely on the nature of the process, the polymer and the heat history of the polymer during production.
It is even possible to build stresses into a polymer sheet by conveying it via rollers rotating at slightly different speeds or by cooling one surface faster than another such as by contact with a roller at a different temperature to the surrounding air.
Where a film or sheet is to be thermoformed or laminated it is desirable that any in-built tendencies to shrink are relaxed before the thermoforming operation begins.